The present invention relates to a process for the production of polycrystalline, translucent sintered tubes from more than 99.5% by weight aluminum oxide powder with a grain size less than 1 micron, to which optionally small amounts of magnesium oxide and zirconium oxide may be added. A mixture is made up with binders and lubricants and this initial material extruded into molded bodies and sintered by mild firing between 900.degree. C. and 1200.degree. C. and firing between 1750.degree. C. and 1900.degree. C. in a hydrogen atmosphere or a vacuum. By means of a special extrusion apparatus and a particular process of plasticizing a substantial improvement of the surface properties of the tubes is obtained. This further results in excellent light transmission and a very high mechanical strength.
DE-OS No. 29 18 729 discloses a process for the production of such polycrystalline, translucent aluminum oxides for use in high pressure gas discharge lamps wherein the sintered body has an average grain size of not less than 20 microns. This process is based on an aluminum oxide powder with a grain size of less than 1 micron and a purity of more than 99.5% by weight, to which optionally the additives MgO, La.sub.2 O.sub.3 and Y.sub.2 O.sub.3 may be added, whereupon the powder is thoroughly mixed and the initial material pressed into a molded body, followed by firing in two stages, i.e. once at 1300.degree. C. to 1500.degree. C. and sintering between 1650.degree. C. and 1900.degree. C. for up to 15 hours in a hydrogen atmosphere or in a vacuum. The heating rate should be at least 200.degree. C./hour between 1400.degree. and 1700.degree. C. in order to improve the surface roughness of the product, which on the sintered product is between 3 and 7 microns. Two firing stages are obviously highly cost intensive and in view of the long holding times, the process is time consuming. Furthermore, the growth of crystals during firing is difficult to control, so that reproducibility is difficult.
A further possibility to increase in-line light transmission is described in U.S. Pat. No. 3,935,495, wherein the sintered surface of the aluminum oxide is treated chemically with a flux. This simultaneously reduces the roughness of the surface, but this process step is complicated and involves high production costs. In addition, this chemical etching reduces the mechanical strength of the tubes.
Further polycrystalline aluminum oxide materials are known exhibiting improved in-line light transmission as the result of the use of a special selection of additives. In particular, West German Patent No. 31 08 677 should be mentioned wherein by the addition of 0.03 to 0.15% by weight MgO and 0.002 to 0.07% by weight of a total amount of ZrO.sub.2 and HfO.sub.2 an improvement of the quality of the sintered tubes is obtained.
Altogether, it has been found that MgO is a necessary component in aluminum oxide ceramics, if a high degree of translucence is to be achieved in the sintering process. In particular, MgO controls grain growth during the later sintering stages. The addition of ZrO.sub.2 or HfO.sub.2 is effected to control the evaporation of magnesium in the sintering phase and to simultaneously prevent the formation of spinel in the final product.
It is further disclosed in DE-0S No. 28 10 128 that a conventional binder and lubricant may be added to the initial mixture in order to make possible the extrusion of a pressed, compact body in the form of a sintered tube. A presintering stage is further necessary to remove the organic components prior to sintering. Accordingly, a mild firing is effected in an oxygen-containing atmosphere at temperatures of approximately 900.degree. to 1200.degree. C.
Another process for the production of translucent sintered aluminum oxide bodies is disclosed in DE-OS No. 31 02 750, wherein in particular the extruding tool is being treated. The additives zirconium oxide, hafnium oxide, cerium oxide and/or magnesium oxide are preferably added to the aluminum oxide powder. The powder is mixed in a known manner with plasticizers corresponding to 12% by weight paraffin and 1% by weight stearin. The density of the formed tubes is between 40 and 70% by weight, preferably 60% by weight, of the theoretical density of the corundum. In the extrusion of the tubes special care is taken to ensure that a favorable flow of material occurs in the extrusion tool. The forming process is described in much detail and is effected in two steps, first the preparation of the cylindrical raw blanks by injection molding and then the final shaping by hot extrusion. The individual tubes are then embedded in aluminum oxide powder to subsequently remove organic substances. Prefiring is effected at 1100.degree. C. for one hour; subsequent sintering is then performed in a vacuum furnace at 1800.degree. C. This method has two disadvantages, in that additional expensive Al.sub.2 O.sub.3 must be used for the embedding and then the adhering powder must be removed in a subsequent work step. Furthermore, contact reactions of the tubes may occur during sintering due to the horizontal position of the tubes. Tubes produced in this manner have a light transmission of 93% and a bending strength of 600 N/mm.sup.2, but since the measuring method used to determine the bending strength is not described, these values cannot be compared with others. Density is around 3.98 g/cm.sup.3 and the average crystal size is approximately 10 microns, which indicates poor in-line light transmission. This forming process eliminates the isostatic presses generally used, but does not produce a sintered tube with optimum properties in view of light transmission and mechanical strength, which are the principal problems encountered in later use as high pressure gas discharge lamps. Fundamentally it may be stated that the problem is the crystal size of the sintered Al.sub.2 O.sub.3 body. While with rising crystal sizes light transmission increases, the mechanical strength is reduced. With declining crystal sizes the two parameters are affected inversely. Furthermore, the plasticizer proportion of 13% paraffin and stearin is high, which is the cause of the appreciable shrinkage, invoking the danger of an excessive number of micropores in the sintered tube. In addition, the straightness of the sintered tube may be affected, if larger amounts of the plasticizer are introduced in an inhomogeneous manner into the aluminum powder. It has been found in principle that in addition to the use of a suitable aluminum oxide powder with the appropriate additives, the sintering process also is important but that primarily the forming process has a significant effect on the sintered end product.